As is known, many integrated circuits employ thin-film resistors usually for controlling the current or the voltage of other components of the integrated circuit. Thin-film resistors are typically deposited by vacuum evaporation or sputtering and are made of different materials, such as CrSi, TaN, and NiCr.
Even using highly controlled processes, the initial resistance of these components may have a tolerance of 3-15%, according to the desired value of resistance. More accurate values may be obtained by physically removing portions of the resistor in a subsequent trimming operation, carried out via laser while verifying the electrical characteristics of components at wafer level, i.e., during electrical wafer sorting (EWS). To this end, laser-beam trimming systems have been developed, which have various advantages, such as speed, accuracy, and cleanliness. These systems may be controlled via computer to modify and adjust the electrical parameters of the components while their values are being measured.
However, resistors are frequently sensitive to temperature. For example, current-reference circuits use load resistors for controlling the current level. As the operating temperature of the circuit changes, the current also changes. Often in devices and circuits where resistor precision is required, large resistance values may also be required because the resistor area may be a controlling factor in determining the device density. Even though, for manufacturing good resistors, materials with high resistivity may be used, they typically have a high temperature coefficient of resistivity (TCR), in the order of about 100 ppm/° C. The TCR is defined as the normalized first derivative of resistance versus temperature and provides an adequate way for measuring the performance of a resistor.
In order to obtain resistors with almost zero TCR, various solutions have been proposed, such as series- or parallel-connected resistors, of different materials having different TCRs, and, more precisely, positive TCRs and negative TCRs matched together so that the effective temperature coefficient is almost zero (see, by way of example, U.S. Pat. No. 7,217,981, which is incorporated by reference).
However, trimming of the TCR may require manufacturing and connecting at least two or more resistors having different TCRs. Consequently, the known solutions are rather complex and require many masks, various materials, and/or various levels of interconnection, and the finished devices that include these resistors are costly.